1. Field of the Invention
This invention relates generally to the fields of thick film and hybrid circuit fabrication. More particularly, the invention provides a perimeter anchored thick film pad that is especially well suited for use in a hybrid circuit.
2. Description of the Related Art
Hybrid circuits utilizing thick film technology are well known. A thick film circuit is fabricated by patterning conductive, resistive and/or dielectric paste materials onto a ceramic substrate in order to form thick film conductors, and possibly also to form passive circuit components, such as resistors and capacitors. The thick film material generally includes a functional component that determines its electrical properties, a binder component that provides adhesion between the thick film material and the ceramic substrate, and a vehicle to establish printing characteristics. Typically, the thick film paste is patterned on the ceramic substrate, dried to evaporate the solvents from the printed film, and fired in an oven. A hybrid circuit may then be fabricated by soldering conventional electronic components (active or passive) to thick film bonding pads printed on the substrate with conductive thick film materials.
The functional component in conductive thick film materials is typically a fine metal powder, such as copper (Cu), gold (Au), silver (Ag), palladium-silver (PdAg), platinum-silver (PtAg), palladium-gold (PdAu), or platinum-gold (PtAu). Bonding pads and circuit traces are typically patterned from such thick film materials by screen printing the conductive metalic paste onto a ceramic substrate, although other patterning methods are also used, such as spin coating or a combination of screen printing and wet chemical etching. A solder mask layer is then typically added to cover the conductive traces and separate adjacent bonding pads. The solder mask layer is generally screen printed in a pattern onto the ceramic substrate and cured, leaving an area of bare substrate between the solder mask layer and the thick film bonding pads. In this manner, the thick film pads are left entirely uncovered by the solder mask layer, and may be coated with a layer of solder by immersing the entire circuit into a solder bath. Because the solder mask layer typically covers everything but the bonding pads, the pads may be coated with solder without causing solder shorts between adjacent pads and traces. A hybrid circuit may then be formed by aligning the connection terminals of a surface-mount component onto the bonding pads, and reflowing the solder layer to form solder joints between the thick film pads and connection terminals.
As surface-mount components and hybrid circuits are made increasingly smaller and more compact, thick film bonding pads must also be made smaller and positioned at a tighter pitch. Decreasing the size of a thick film bonding pad, however, typically results in less adhesion between the pad and the ceramic substrate. The level of adhesion between the bonding pad and substrate is commonly referred to by those skilled in the art of thick film and hybrid circuit fabrication as peel strength. The peel strength of a thick film bonding pad is typically affected by the size of the pad and by thermal stress induced during the fabrication process. In addition, soldering electronic components to the thick film bonding pads causes additional thermal stress, which further reduces the peel strength, and commonly results in mechanical failure of the bonding pad.
A thick film circuit with a perimeter anchored thick film pad is provided. The thick film circuit includes a base substrate, a thick film bonding pad, and a solder mask layer. The thick film bonding pad is formed on the surface of the base substrate. The solder mask layer is also formed on the surface of the base substrate, and overlaps a portion of the thick film bonding pad in order to improve adhesion between the thick film bonding pad and the base substrate.